As wheels for supporting tires required for automobiles to travel on, there have widely been used two-piece wheels comprising a wheel rim (hereinafter also referred to simply as “rim”) in the form of a hollow cylindrical body and a disk-shaped wheel disk (hereinafter also referred to simply as “disk”) inserted in the wheel rim, the wheel rim and the wheel disk being joined to each other by MIG welding, TIG welding, or the like. In recent years, it is a mainstream trend to make both a rim and a disk of aluminum to meet demands for lightweight automobiles.
The disk is manufactured by machining a plate-like aluminum blank such as a an extended aluminum member by drawing, and thereafter forming a hub hole, bolt holes, and ornamental holes for improved design and heat radiation in the aluminum blank by punching or cutting.
The rim is manufactured as follows: First, the end faces of an elongate rectangular plate are brought into abutment against each other, and thereafter the abutting end faces are joined to each other by resistance welding, MIG welding, or the like, thereby forming a hollow cylindrical body.
Then, the welded region of the hollow cylindrical body is trimmed or cut to remove edges, after which the hollow cylindrical body is rolled by a multi-step rolling process (see Patent Document 1, for example), forming a recess called a drop portion 2 in a substantially central region of an outer circumferential wall of the hollow cylindrical body 1, as shown in FIG. 42. The reference numeral 3 in FIG. 42 represents a welded seam.
After curled portions are formed on the opposed ends of the hollow cylindrical body 1, hump portions directed from an inner circumferential wall toward the outer circumferential wall of the hollow cylindrical body 1 are formed, thereby producing a rim.
In the process of forming the drop portion 2, the welded seam 3 may crack, as described in Patent Document 2. If the welded seam 3 cracks, then the production efficiency of the rim is lowered because the cracked welded seam 3 needs to be repaired. According to Patent Document 2, it has been proposed to heat the welded seam 3 to substantially equalize the hardness thereof to the hardness of the other regions, so that the welded seam 3 is prevented from being cracking.
In order to increase the strength of the rim, the ends of the rim may be bent into curled portions, as described in Patent Document 3.
The disk is inserted into the rim thus manufactured, and they are joined to each other by arc welding, thereby producing a wheel.
For joining the disk and the rim to each other by arc welding, the wheel is inclined 30° to the horizontal direction, and the welding torch is aimed at a position that is closer to the disk by a distance corresponding to the diameter of the welding wire. The welding current and voltage, and the moving speed of the welding torch are adjusted depending on the thicknesses of the rim and the disk, for thereby forming a welded bead in the range of about 10 to 30% of the thickness of the rim (see Patent Document 4).
Patent Document 1: Japanese Laid-Open Patent Publication No. 2-70304;
Patent Document 2: Japanese Laid-Open Patent Publication No. 63-224826;
Patent Document 3: Japanese Laid-Open Utility Model Publication No. 63-56935; and
Patent Document 4: Japanese Laid-Open Patent Publication No. 5-58103.